The present invention is related in general to the field of semiconductor devices and processes and more specifically to the structure, materials and fabrication of leadframes for integrated devices.
The leadframe for semiconductor devices was invented (U.S. Pat. No. 3,716,764 and U.S. Pat. No. 4,034,027) to serve several needs of semiconductor devices and their operation simultaneously: First of all, the leadframe provides a stable support pad for firmly positioning the semiconductor chip, usually an integrated circuit (IC) chip. Since the leadframe including the pads is made of electrically conductive material, the pad may be biased, when needed, to any electrical potential required by the network involving the semiconductor device, especially the ground potential.
Secondly, the leadframe offers a plurality of conductive segments to bring various electrical conductors into close proximity of the chip. The remaining gap between the (xe2x80x9cinnerxe2x80x9d) tip of the segments and the conductor pads on the IC surface are typically bridged by thin metallic wires, individually bonded to the IC contact pads and the leadframe segments. Obviously, the technique of wire bonding implies that reliable welds can be formed at the (inner) segment tips.
Thirdly, the ends of the lead segment remote from the IC chip (xe2x80x9couterxe2x80x9d tips) need to be electrically and mechanically connected to xe2x80x9cother partsxe2x80x9d or the xe2x80x9coutside worldxe2x80x9d, for instance to assembly printed circuit boards. In the overwhelming majority of electronic applications, soldering accomplishes this attachment. Obviously, the technique of soldering implies that reliable wetting and solder contact can be performed at the (outer) segment tips.
It has been common practice to manufacture single piece leadframes from thin (about 120 to 250 xcexcm) sheets of metal. For reasons of easy manufacturing, the commonly selected starting metals are copper, copper alloys, iron-nickel alloys for instance the so-called xe2x80x9cAlloy 42xe2x80x9d), and invar. The desired shape of the leadframe is etched or stamped from the original sheet. In this manner, an individual segment of the leadframe takes the form of a thin metallic strip with its particular geometric shape determined by the design. For most purposes, the length of a typical segment is considerably longer than its width.
After assembly on the leadframe, most ICs are encapsulated, commonly by plastic material in a molding process. It is essential that the molding compound, usually an epoxy-based thermoset compound, has good adhesion to the leadframe and the device parts it encapsulates. Palladium, described above as the outermost layer of the leadframe, offers excellent adhesion to molding compounds.
The leadframe not only has to tolerate an encapsulation process at elevated temperatures, but also should be amenable to good adhesion to the encapsulating material wherever the leadframe and the encapsulating material share a common boundary. The adhesion should withstand thermomechanical stresses and prevent the ingress of unwanted moisture and chemicals.
Experience has shown that large leadframe chip pads, introduced for supporting large area chips, have a strong tendency to delaminate from the chips, which have been attached to one pad surface by polymeric materials, and also from commonly used encapsulation materials, which surround the outer pad. The small voids thus created allow the accumulation of water molecules such that in the course of few days films of water are formed within the voids. Alternating between liquid and gaseous phases during the wide temperature swings encountered in accelerated testing, board solder attachment, and device operation, these water accumulations exert enough force on the encapsulating material to aggravate the delamination and eventually to originate microcracks. Quickly propagating, these nascent cracks frequently lead to destruction of the assembled device (known in the literature as xe2x80x9cpopcorn effectxe2x80x9d).
Numerous proposals have been discussed in the literature to avoid delamination and popcorn effect by modifying the design of the leadframe and/or the surface preparation of the leadframe material. A preferred approach is to reduce the area of the chip mount pad so that the encapsulation material obtains direct contact and thus strong adhesion to the passive surface of the chip. Examples are described in U.S. Pat. No. 5,233,222 of Aug. 3, 1993 (Djennas et al., xe2x80x9cSemiconductor Device having Window-Frame Flag with Tapered Edge in Openingxe2x80x9d); U.S. Pat. No. 5,327,008 of Jul. 5, 1994 (Djennas et al., xe2x80x9cSemiconductor Device having Universal Low-Stress Die Support and Method for Making the samexe2x80x9d); U.S. Pat. No. 5,424,576 of Jun. 13, 1995 (Djennas et al., xe2x80x9cSemiconductor Device having X-Shaped Die Support Member and Method for Making the samexe2x80x9d); U.S. Pat. No. 5,429,992 of Jul. 4, 1995 (Abbott et al., xe2x80x9cLeadframe Structure for IC Devices with Strengthened Encapsulation Adhesionxe2x80x9d); U.S. Pat. No. 5,610,437 of Mar. 11, 1997 (Frechette, xe2x80x9cLeadframe for Integrated Circuitsxe2x80x9d); U.S. Pat. No. 5,633,528 of May 27, 1997 (Abbott et al., xe2x80x9cLeadframe Structure for IC Devices with Strengthened Encapsulation Adhesionxe2x80x9d); and U.S. Pat. No. 5,714,792 of Feb. 3, 1998 (Przano, xe2x80x9cSemiconductor Device having a Reduced Die Support Area and Method for Making the samexe2x80x9d).
The undesirable consequence of all these approaches is the fact that the reduced area of the chip mount pad significantly increases the length of the straps connecting the mount pad to the leadframe. Consequently, these straps end up mechanically weakened. The temperature excursions during and after the molding process induce deformations, which frequently lead to failure of the straps due to shifting and tilting.
An urgent need has therefore arisen for a low-cost, reliable design approach for IC leadframes which provides all the assembly features leadframes are expected to offer: Immunity to the thermomechanical stresses in the molding process, adhesion to polymeric compounds, bondability and solderability. The new leadframe and its method of fabrication should be flexible enough to be applied for different semiconductor product families and a wide spectrum of design and assembly variations, and should achieve improvements toward the goals of improved process yields and device reliability. Preferably, these innovations should be accomplished using the installed equipment base so that no investment in new manufacturing machines is needed.
According to the present invention for the structure of a semiconductor integrated circuit (IC) leadframe, the chip mount pad has an area smaller than the chip intended for mounting and a plurality of support members, each attached externally to the perimeter of the pad and internally to the leadframe, and each having at least one portion located within the perimeter of the chip in a configuration operable to accommodate bending and stretching beyond the limit of simple elongation based upon inherent material characteristics.
The present invention is related to high density ICs, especially those having high numbers of inputs/outputs, and also to low end, low cost devices. These ICs can be found in many semiconductor device families such as standard linear and logic products, digital signal processors, microprocessors, digital and analog devices, high frequency and high power devices, and both large and small area chip categories. The package type can be plastic dual in-line packages (PDIPs), small outline ICs (SOICs), quad flat packs (QFPs), thin QFPs (TQFPs), SSOPs, TSSOPs, TVSOPs, and other leadframe-based packages.
It is an aspect of the present invention to provide a leadframe having the mount pad support members designed so that they absorb thermomechanical stress exerted during the molding encapsulation process, accelerated testing involving temperature and moisture variations, and the device operation.
Another aspect of the present invention is to design the geometries of the pad support members so that they include portions which provide bending and stretching beyond the limit of simple elongation based upon inherent material characteristics.
Another aspect of the present invention is to avoid deformation, shifting or tilting of the pad support members during the molding process.
Another aspect of the present invention is to design the pad support members so that at least two of them intersect at a point near the center of the chip mount pad.
Another aspect of the invention is to maximize the stress-absorbing feature of the pad support members by designing the thickness of the intersecting members greater than the remaining portions of the members.
Another aspect of the invention is to reach these goals without the cost of equipment changes and new capital investment, by using the installed fabrication equipment.
These aspects have been achieved by the teachings of the invention concerning the geometries, the intersection and the thickness of the support members, and have been applied to whole families of IC chips of various chip areas.
In the first embodiment of the invention, the chip mount pad support members provide portions having a meandering or sinusoidal geometry within the perimeter of the chip. The design of these support member portions contributes 50 to 500% over the contribution of the material elongation to the support member stretching.
In the second embodiment of the invention, the support member portions of the first embodiment, located within the perimeter of the chip, are enhanced by additional support member portions of meandering geometry, located outside the perimeter of the chip.
In the third embodiment of the invention, the chip mount pad support members intersect at a point near the center of the chip mount pad. In a preferred embodiment, the thickness of the intersecting members is greater than the thickness of the remaining portions of the support members.
The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.